The Difference Between Enterprise & Client SSD – Kingston Technology

Endurance

All NAND Flash memory contained in Flash storage devices degrade in their ability to reliably store bits of data with every program or erase (P/E) cycle of a NAND Flash memory cell until the NAND Flash blocks can no longer reliably store data; at that point, a degraded or bad block is removed from the user addressable storage pool and the logical block address (or LBA) is moved to a new physical address on NAND Flash storage array. A new storage block replaces the bad one using the Spares Block pool that is part of the Over Provisioned (OP) storage on the SSD.

As the cell is constantly programmed or erased, the BER also increases linearly and it is for this reason a complex set of management techniques must be implemented on the enterprise SSD Controller to manage the cell capability to reliably store data over the expected life of the SSD.{{Footnote.N52083}}

The P/E endurance of a given NAND Flash memory can vary substantially depending on the current lithography manufacturing process and type of NAND Flash produced.

NAND flash memory typeTLCMLCSLC

Architecture
3 bits per cell
2 bits per cell
1 bit per cell

Capacity
Highest capacity
High capacity
Lowest capacity

Endurance (P/E)
Lowest endurance
Medium endurance
High endurance

Cost
$
$$
$$$$

Approx NAND Bit Error Rate (BER)
10^4
10^7
10^9

Table 2 – NAND flash memory types {{Footnote.N52084}}{{Footnote.N52085}}

Enterprise SSDs will also vary from client SSDs on their duty cycle. An enterprise class SSD must be able to withstand heavy read or write activity in scenarios typical with a datacenter server requiring access to the data across the entire 24 hours of every day in the week compared to a Client class SSD which is typically only fully utilized for 8 hours a day in the week. Enterprise SSDs have a 24×7 duty cycle compared to client SSDs with a 20/80 duty cycle (20% of the time active, 80% in idle or sleep mode during computer usage).

Understanding the write endurance of any application or SSD can be complex, which is why the JEDEC committee also proposed an endurance measurement metric using the TeraBytes Written (TBW) value to indicate the amount of raw Host data that can be written to the SSD before the NAND Flash contained in the SSD becomes an unreliable storage medium and the drive should be retired.

Using the JEDEC proposed JESD218A testing methods and JESD219 enterprise class workloads, it becomes an easier task to interpret an SSD manufacturer’s endurance calculations via TBW and extrapolate a more understandable endurance measure that can be applied to any datacenter.

As noted in documents JESD218 and JESD219, different application class workloads can also suffer from a Write Amplification Factor (WAF) in order of magnitude higher than the actual writes submitted by the host and easily lead to unmanageable NAND Flash wear, higher NAND Flash BER from excessive writes over time and slower performance from widely distributed invalid pages across the SSD.

While TBW is an important topic for the discussion between enterprise and client class SSDs, TBW is only a NAND Flash level endurance prediction model and the Mean Time Between Failure (MTBF) should be observed as a component level endurance and reliability prediction model based on the reliability of components utilized on the device. The expectation of an enterprise class SSDs components include outlasting and working harder at managing the voltages across all NAND Flash memory over the SSDs life expectancy. All enterprise SSDs should be rated at least at one million hours MTBF, which translates to over 114 years! Kingston specs its SSDs very conservatively and it is not uncommon to see higher MTBF specifications on SSDs; it is important to note that 1 million hours is more than a sufficient starting point for enterprise SSDs.

S.M.A.R.T. monitoring and reporting on enterprise class SSDs allows the device to be easily queried pre-failure for life expectancy based on the current write amplification (WAF) factor and wear level. Pre-failure predictive warnings for failure events such as a loss of power, bit errors occurring from the physical interface or un-even wear distribution are often also supported. The Kingston SSD Manager utility can be downloaded from the Kingston web site and used to view a drive’s status.

Client class SSDs may only feature the minimum S.M.A.R.T. output for monitoring the SSD during standard use or post-failure.

Depending on the application class and capacity of the SSD, an increased reserve capacity of NAND Flash memory can also be allocated as an over-provisioned (OP) spare capacity. The OP capacity is hidden from user and operating system access and can be utilized as a temporary write buffer for higher sustained performance and as a replacement of defective Flash memory cells during the life-expectancy of the SSD to enhance the reliability and endurance of the SSD (with greater numbers of Spare Blocks).